As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks have to project smaller and smaller structures. In order to fulfil this demand, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In order to use significantly smaller wavelengths, lithography systems for the extreme ultraviolet (EUV) wavelength range (approximately 10-16 nm) are presently in development.
Photolithographic masks have to fulfil highest demands with respect to transmission, planarity, pureness and temperature stability. In particular, the surface of reflective masks for EUV radiation coated with the reflective structure has to be plane within the range of about 1 nm in order to avoid aberrations of the desired structure in the photoresist of the wafer. These challenges also apply for other EUV reflective optical elements, as for example mirrors used in the beam path of EUV lithography systems.
The above mentioned challenges require highly precise techniques for the production of the substrates of EUV optical elements. However, even the best production techniques cannot guarantee surface variations below 1 nm. Moreover, the fabrication of mask blanks and/or EUV optical elements from mask blanks may additionally induce further defects in the EUV substrates, and/or thus also in the EUV optical elements. It is therefore necessary to correct defects of EUV optical elements in order to establish an economical production process for these components.
On the other hand, an extremely careful and precise handling and holding of EUV mask blanks and/or EUV optical elements is necessary in order to avoid as far as possible mechanical abrasion and/or the formation of particles from the EUV optical element which may deteriorate the function of an EUV lithography system. Since an EUV optical element is used to expose a large number of semiconductor substrates or wafers, a high effort in terms of production and handling of EUV optical elements is almost always justified.
In order to fulfil these handling requirements, mask EUV blanks are held on an electrostatic chuck during the fabrication of an EUV optical element. Further, EUV masks are also held with an electrostatic chuck in the lithography system during the wafer illumination. As the substrate of EUV optical elements typically comprises a dielectric material or a semiconducting material, an electrically conducting layer has to be deposited on the rear side of a substrate in order to be able to hold the substrate with an electrostatic chuck during the fabrication and/or operation of the optical element.
The US 2006/0115744 A1 discloses a method for producing of a mask blank having an electrically conducting layer on a rear side of the substrate of an EUV photomask. The metallic layer has a layer thickness of around 100 nm. The abrasion resistance of the metallic layer have been investigated by comprehensive abrasion tests in order to check whether a mask blank coated on the rear side with an electrically conducting layer can be handled with electrostatic chucks without the risk of mechanical abrasion.
As already mentioned, errors already introduced in the substrate during the substrate production and/or introduced during the fabrication process of the EUV optical element have to be corrected at the end of the production process of the EUV optical element. Moreover, defects may evolve in the course of the operation of an EUV mask in a lithography system.
It is already known that a surface of an EUV optical element can be modified in a controlled manner in order correct planarity and/or placement defects by applying ultra-short laser pulses into the substrate of an optical element (cf. U.S. Pat. No. 6,841,786 B2, DE 10 2006 054 820 A1, U.S. Ser. No. 13/179,799 A1, U.S. Pat. No. 6,844,272 B2, U.S. Pat. No. 6,821,682 B2, US 2007/0 224 522, and US 2008/0 033 206).
This defect compensation occurs through the rear side of the EUV optical element as the ultra-short laser pulses cannot penetrate the multi-layer structure, which forms the reflective optical element arranged on the front surface of the EUV optical element. Consequently, the electrically conducting layer deposited on the rear side for holding the EUV optical element with an electrostatic chuck has also to be optically transparent for the ultra-short laser pulses.
The European Patent Application EP 2 317 582 A1 discloses a thin layered structure as an electrode for optoelectronic devices, wherein the thin layered structure is also optically transparent. The thin layered structure comprises at least one thin metal film and at least one ultra-thin metal film, wherein the two films or layers have different materials.
The European Patent specification EP 2 133 921 B1 describes an ultra-thin metallic transparent electrode which is thermally treated in an ambient atmosphere or optionally in combination with an oxygen treatment in order to make the ultra-thin coating stable against environmental stress.
As being held on an electrostatic chuck, rear side coatings of EUV optical elements have in addition of being electrically conducting and optically transparent also to fulfil specific mechanical requirements. For example, the pins of an electrostatic chuck or particles may indent in the surface coating on the substrate rear side. Moreover, the rear side coating has to withstand the lateral accelerations occurring during the mask scanning process. For this reason, as already explained in the US 2006/0115744 A1, the coating on the rear side of the substrate of an EUV optical element has to withstand abrasion during the handling of mask blank and/or the EUV optical element with an electrostatic chuck. Further, the electrical conductivity of the rear side coating has to be high enough, so that the mask blank and/or the EUV optical element can securely be handled with an electrostatic chuck. Moreover, the rear side coating has to be optically transparent, so that ultra-short laser pulses with a high optical intensity can be applied through the coating into the substrate of the mask blank and/or the EUV optical element.
It is therefore one object of the present invention to provide a coating and a method for depositing the coating on a substrate of a photolithographic mask that is electrically conducting, optically transparent and additionally has suitable mechanical properties.